Ink jet printing is a non-impact process in which digital signals produce droplets of ink on substrates such as paper or transparency films. Pigments are preferred ink colorants due to their lightfastness and their potential to offer waterfastness and resistance to smear and smudge. The pigmented ink, however, must be stable to flocculation and settling throughout the life of the ink.
In thermal ink jet printing, resistive heating is used to vaporize the ink, which then is expelled through a printhead orifice onto a substrate. Composition of the ink changes dramatically during the process, which tends to cause pigment particles to flocculate in or near the printhead orifices. Ink drops may be misdirected as a result, and in extreme cases the orifices may become plugged.
The ink typically contains many organic additives (e.g., thickeners and thixotropic agents) to adjust its physical properties. Organic cosolvents generally are included to improve ink penetration into the printing substrate, and surfactants typically are added to decrease ink dry-time after printing. As the ink dries, concentration of these components increases, jeopardizing stability of the pigment dispersion that is necessary to obtain uniformity and color quality of the printed image.
Pigment dispersions generally are stabilized by either a non-ionic or ionic technique. When the non-ionic technique is used, the pigment particles are stabilized by a polymer having a hydrophilic section that extends into the water medium, providing entropic or steric stabilization. Polyvinyl alcohol, cellulosics, ethylene oxide modified phenols, and ethylene oxide/propylene oxide polymers may be used for this purpose. While the non-ionic technique is not sensitive to pH changes or ionic contamination, it has a major disadvantage in that the printed image is water sensitive.
In the ionic technique, the pigment particles are stabilized using the polymer of an ion containing monomer, such as neutralized acrylic, maleic, or vinyl sulfonic acid. The polymer provides stabilization through a charged double layer mechanism whereby ionic repulsion hinders the particles from flocculation. Since the neutralizing component tends to evaporate after printing, the polymer then has reduced water solubility and the printed image is not water sensitive.
Dispersants having random, block, and graft polymeric structures have been proposed in the art. U.S. Pat. No. 4,597,794 to Canon discloses aqueous ink dispersions wherein the pigment particles are dispersed using a polymer having ionic hydrophilic segments and aromatic hydrophobic segments that adhere to the pigment surfaces. U.S. Pat. No. 5,085,698 to DuPont discloses use of AB and BAB block polymer dispersants. Unexamined Japanese application JP6-100,810 to Canon discloses using certain graft copolymers having a hydrophilic portion containing acid groups and a hydrophobic portion primarily composed of styrenes and alkyl esters of meth(acrylic acid).
While random polymeric dispersants such as those proposed in U.S. Pat. No. 4,597,794 can be prepared readily using conventional polymerization techniques, structured polymeric dispersants such as those taught in U.S. Pat. No. 5,085,698 usually provide better dispersion stability. The structured polymers, however, are more difficult to manufacture and require raw materials having a high purity. The graft copolymers proposed in JP6-100,810 are prepared in an elaborate multi-step process generally requiring purification steps before the macromonomers can be used in the synthesis of the final graft copolymer.
Accordingly, there is an ongoing need for improved aqueous pigmented inks for ink jet printing using dispersants that are easily prepared and that provide excellent stability of the ink during the demanding conditions of ink jet printing.